Electrochemical sensors for the measurement of gas concentrations are well known. Working, counter, and reference electrodes are connected through a potentiostat circuit, the main purpose of which is to set the bias potential of the working electrode through the reference electrode potential.
The working electrode (also called the sensing electrode) is designed to optimise the oxidation or reduction of the gas to be measured. This electrode allows the gas to come in contact with both electrocatalyst and electrolyte to create a three-phase interface of gas, liquid and solid. The other two electrodes in the cell, the counter electrode and the reference electrode usually have a similar chemical composition to the working electrode. Oxidation or reduction at the working electrode generates a current that is generally linearly proportional to the amount of gas oxidised or reduced; this current is therefore also proportional to the concentration of the gas at the electrode.
The cell electrolyte provides ionic electrical contact between the electrodes, usually with the aid of hydrophilic separators to allow capillary transport of the electrolyte.
The counter electrode balances the reaction of the working electrode—if the working electrode oxidises the gas, then the counter electrode must reduce some other molecule to generate an equivalent current, in the opposite sense. For example, where carbon monoxide will be oxidised on the working electrode, oxygen will usually be reduced on the counter electrode. Unlike the working electrode, the counter electrode potential is allowed to vary. The counter electrode potential in clean air is close to the working electrode, but as current is demanded from the counter electrode, the potential increases, so the secondary responsibility of the potentiostat circuit is to ensure that adequate current is fed to the counter electrode and that the counter electrode can operate at its preferred potential.
An important feature of conventional sensors is that certain gases may interfere with the desired working of the sensor. For example, in a sensor intended to measure carbon monoxide concentration, hydrogen sulfide may also react at the working electrode and generate a current indistinguishable from that generated by carbon monoxide. Particular interferents will depend on the particular form of sensor and the intended gas to be measured; the skilled person will be aware of these. To reduce such interference, it is generally necessary to isolate the sensor from the interferent gas; for example, carbon monoxide sensors are typically fitted with ‘scrubbers’, or chemical filters which allow carbon monoxide to pass but which prevent hydrogen sulfide from reaching the working electrode. While this arrangement is generally effective, this does mean that separate sensors are necessary to detect separate gases which may otherwise interfere with one another. This will increase the size and cost of any apparatus which requires sensors to detect multiple gases.